Vsphere Ha Slot Size

If you have the VM configured with the highest memory reservation of 8192 MB (8 GB) and highest CPU reservation of 4096 MHZ. Among the other VM’s in the cluster, then the slot size for memory is 8192 MB and slot size for CPU is 4096 MHZ. VMware vSphere 4.1 - HA Admission Control Slot Calculation. Posted on 07 Feb 2011 by Ray Heffer. VMware HA (High Availability) admission control is something I wanted to understand better so I started making notes gathered from various sources on the subject, and in particular the way slot sizes are calculated. Learn what a slot is, how it is calculated, and how it affects your vSphere HA Admission Control.

Vsphere Ha Slot Size Chart

By Duncan Epping, Principal Architect, VMware

Yesterday I received a question on twitter:

Vsphere Ha Slot Size Calculation

Hi, to settle an argument in the office, if no reserves are in place, does number of vCPU’s affect slot size in vSphere 4? Thx 🙂

First of all, what is a slot? The availability guide explains it as follows

A slot is a logical representation of the memory and CPU resources that satisfy the requirements for any powered-on virtual machine in the cluster.

In other words a slot is the worst case CPU and Memory reservation scenario for any given virtual machine in a cluster. This slot is used when Admission Control is enabled and “Host Failures Tolerates” has been selected as the admission control policy. The total amount of available resources in the cluster will be divided by the slot size and that dictates how many VMs can be powered on without violating availability constraints. Meaning that it will guarantee that every powered on virtual machine can be failed over.

As said this slot is dictated by the worst case reservation for CPU and Memory. Prior to vSphere 4.0 we used the number of vCPUs to determine the slotsize for CPU as well. But we do not use vCPUs anymore to determine the slot size for CPU. The slotsize for CPU is determined by the highest reservation or 256MHz (vSphere 4.x and prior) / 32MHz (vSphere 5) if no reservation is set.

However, vCPUs can have an impact on your slot… it can have an impact on your memory slotsize. If no reservation is set anywhere HA will use the highest Memory Overhead in your cluster as the slot size for memory. This is where the amount of vCPUs come in to play, the more vCPUs you add to a virtual machine the higher will your memory overhead be.

I guess the answer to this question is: For CPU the number of vCPUs does not impact your slotsize, but for memory it may.